Ultrasonic study of liquids and liquid mixtures has gained much importance
during the last two decades in assessing the nature of molecular interactions
and investigating the physicochemical behavior of these systems. Molecular association
is very well studied by several workers using ultrasonic method especially in
the case of alcohols. Alcohols are polar liquids (Venkatesu
et al., 2006), strongly self-associated by hydrogen bonding to the
extent of polymerization that may differ depending on temperature, chain length
and position of the OH group and dilution by other substances. As hexane is
a nonpolar chain molecule, the alcohol molecules associate with hexane medium
and form clusters (Crossley, 1971). Liquid tertiary amines
(Wolff and Gamer, 1972; Landeck et
al., 1977; Cibulka and Nagata, 1987; Wolff
et al., 1995; Matthias Kwaterski et al.,
2006; Megiel et al., 2001) are weakly polar,
non-associated, strong proton acceptors. It is well known, that mixture containing
associating components like alkanols and amines are highly non-ideal systems.
Due to the formation of hydrogen bonds between the different species large negative
heats as well as volumetric effects are observed upon mixing. Therefore in order
to have a clear understanding of the intermolecular interactions between the
1-alkanol + n-hexane + TBA molecules an attempt has been made to measure density,
viscosity and ultrasonic velocity for the mixtures at three different temperatures
over the entire composition range. From the experimental values, a few acoustical
parameters such as adiabatic compressibility ( β), intermolecular free length
(Lf), free volume (Vf), internal pressure ( πi)
and their excess parameters have been calculated. These parameters are found
to be sensitive in exploring the interactions between the component molecules,
which enable us to have a better understanding of the liquid mixtures.
MATERIALS AND METHODS
The chemicals (AR-grade) 1-pentanol, 1-hexanol, n-hexane and Tri-n-butyl amine
(TBA) used in the present study were purified as per standard procedures (Vogel,
The density of the various systems at different temperatures 303, 308
and 313 K has been measured using relative measurement method and the
viscosity of the mixtures was measured using an Ostwalds viscometer.
The ultrasonic velocity of the liquids mixtures are measured using a
single crystal variable path interferometer at 3 MHZ (Model M81) supplied
by Mittal Enterprises, New Delhi, India. The temperature of the liquid
mixtures was maintained constant by circulating water from a thermostatically
controlled water bath with an accuracy of ±0.01 K.
From the measured value of the ultrasonic velocity (U), density (ρ)
and viscosity ( η), the following acoustical parameters were calculated:
The values of KT for different temperatures were taken from the
work of Jacobson (1952), where KT is a temperature
where, Meff is the effective molecular weight, K is a temperature
independent constant equal to 4.28x109 for all liquids.
where, b stands for the cubic packing factor which is assumed to be 2
for all liquids and solutions, R is the gas constant and T is the absolute
Excess parameter AE by definition represent the difference
between the parameter of real mixture (Aexp) and those corresponding
change to an ideal mixture (Aid) as:
Aid = ΣAi Xi, where Ai
is any acoustical parameter and Xi is the mole fraction of
the liquid component.
RESULTS AND DISCUSSION
From Table 1, it can be seen that the density, viscosity
and ultrasonic velocity increase with increase in mole fraction of alcohols
and the same is decrease with increase of temperature. The increasing
values of ρ, η and U with show that there is a moderate attraction
between solute and solvent molecules. The decrease of values with temperature
shows a decrease in intermolecular forces due to increasing the thermal
energy of the system.
||Values of density (ρ), viscosity ( η) and velocity
|| Values of adiabatic compressibility ( β), free length
(Lf), free volume (Vf) and internal pressure ( πi)
The primary alkanols are having a characteristic carbo cation. The stability
of a charged system is increased by the dispersal of the charge. Hence,
any factor that tends to spread out the positive charge of the electron
deficient carbon and distribute it over the rest of the ion must stabilize
the carbo cation. Further, this carbo cation will be stabilised by electron
donating substituents. The alkyl group of alcohols attached to the carbon
atom bearing positive charge exerts an electron-releasing inductive effect
and this reduces the positive charge of the carbon atom to which it is
attached, in doing so, the alkyl group itself becomes positive. Therefore
dipole-dipole interaction or hydrogen bonding is expected between the
The free length Lf dependence on the adiabatic compressibility
and show a similar behavior to that of the compressibility and inverse
to that of velocity. It decreases with increase in concentration of alcohol
for both the systems, indicating that there is a significant interaction
between solute molecules, due to which structural arrangements are considerably
affected in Table 2.
System I-Mole fraction vs excess values of adiabatic
compressibility, free length, free volume and internal pressure,
System II-Mole fraction vs excess values of adiabatic
compressibility free length, free volume and internal pressure,
Free volume of the system decrease whereas the internal pressure increases
with increase in concentration of alcohol. This suggests the close packing of
the molecules inside the shield, which may be brought about by the increasing
magnitude of interactions (Mecke, 1950).
The excess functions are found to be more sensitive towards intermolecular
interactions in liquid mixtures. Figure 1a and 2a
shows positive values of βE suggest that rupture of the
associated structure of the alcohols dominates the hydrogen-bond interactions
between like molecules. It is interesting to note that the values of βE
become more positive as the carbon chain length increases, suggesting
the strength of hydrogen bond between TBA and alcohol molecules should
follow the order of 1-pentanol < 1-hexanol.
The change of LfE (Fig. 1b,
2b) from to increasingly negative excess values shows,
greater strength of interaction between the components and may be qualitatively
interpreted in terms of closer approach of unlike molecules leading to
reduction in volume and compressibility.
The values of VfE (Fig. 1c,
2c) are negative and decrease with increase in the
concentration of alcohols, which indicate the presence of strong molecular
interaction and the magnitude of VfE indicates the
formation of complex between the hetero molecules of the mixtures. Increase
in the internal pressure gives rise to the negative values of πiE
(Fig. 1d, 2d) as its dilution causes
disruption of aromatic C-H bond stretching.
Thus the ultrasonic study of the liquid mixtures serves as a probe to
detect the molecular association arising from the hydrogen bonding between
nitrogen atoms of Tri-n-butyl amine (TBA) and OH group of alcohol molecules.
The non-linear variation of acoustical parameters with concentration reveals
the complex formation which is also strongly supported by the excess parameters
in the ternary liquid mixtures.